The present invention relates to a heat treatment technique used in the field of semiconductor devices and particularly to a heat treatment method and a heat treatment apparatus for controlling the temperature of a substrate surface.
In recent years, as high integration of semiconductor devices has progressed, patterns have been miniaturized and preciseness thereof has been improved. In steps of manufacturing a photomask used for pattern exposure, a means for subjecting a photosensitive material to a heat treatment is necessary for the step of forming a pattern. In the process including the step of heating or cooling substrates to be exposed, variants of the treatment temperature are reflected on variants of the pattern size. Therefore, more precise management of the temperature has been demanded in accordance with miniaturization and improvement of the preciseness.
Conventionally, a thermocouple or a resistance bulb is embedded near the surface of a heating plate used for a heating treatment to measure the temperature, when managing the temperature of a substrate. Further, the output of a heating mechanism is controlled based on the obtained temperature, and a heat treatment is performed. However, since quartz forming the under layer of the substrate to be exposed has a very low thermal diffusivity, temperature of the photosensitive material film on the surface of the substrate to be processed cannot be accurately controlled by the method described above.
To perform precise temperature control with user of a quartz substrate used for a substrate to be exposed, it is necessary to measure directly the temperature of the film as a target to be subjected to a heat treatment on the substrate. Means for measuring the surface temperature are roughly divided into two types, one being a contact type and the other being a non-contact type, depending on the characteristics of a monitoring method.
As the contact type, there is a method of measuring the surface temperature by embedding a thermocouple into a film as a target on the substrate. However, it is not a realistic way to embed thermocouples into all substrates to be processed. Also, a photomask manufactured and processed by embedding directly a thermocouple in the substrate to be processed is not suitable for practical use. By attaching a thermocouple or a resistance member, the temperature characteristic is changed. Therefore, there is a problem that the photosensitive material film as a target to be heated cannot be accurately measured. In particular, it is impossible to embed a temperature sensor into a photosensitive material film having a film thickness of about 0.1 to 2 xcexcm called a resist.
From the reasons described above, it is preferable to use the temperature measuring method of the non-contact type. A radiation thermometer is a representative example of the temperature measuring device of this type. There is an example using the radiation thermometer which has succeeded in measurement of a surface temperature of a steel material having a precise oxide film processed by a shape steel line or the like.
However, the temperature measurement of a substrate to be processed or the like with use of the radiation thermometer involves the following problems. In this measurement, since the temperature of the temperature measurement area is as low as about 50 to 200xc2x0 C., the signal is weak. Therefore, if other radiated light than that from the target to be measured enters into the sensor section, a measurement error is caused. In addition, it is not possible to neglect a measurement error due to drifting of the sensitivity which is considered as depending on the environmental temperature and the like. From the reasons described above, the radiation thermometer involves a problem that accurate temperature measurement is difficult.
In addition, to obtain more precise patterns, it is highly necessary to use a step of measuring the temperature of the substrate surface with high preciseness and of performing heating (or cooling) based on the measurement result. Therefore, demands for a method and an apparatus, by which temperature measurement is carried out while monitoring the surface temperature of a substrate (in-situ) and feedback is immediately reflected on a heat control section or the like, have become more and more serious.
The present invention has been made in view of the above-described situation, and has an object of providing a heat treatment method and a heat treatment apparatus, by which the surface temperature of a substrate to be processed such as a substrate for exposure can be measured with high preciseness so that accurate temperature management of a thin film formed on the substrate can be achieved.
To achieve the above object, the present invention according to a first aspect thereof has the structure as follows. A heat treatment method is characterized by comprising steps of: measuring a temperature of a thin film formed on a substrate to be processed, by a radiation thermometer which performs measurement with use of a wavelength range except for a wavelength range of light which is transmitted through the thin film; and controlling a heated temperature of the thin film in response to the temperature measured by the step.
Also, to achieve the above object, the present invention according to the second aspect thereof has the structure as follows. A heat treatment apparatus is characterized by comprising: a support member for supporting a substrate to be processed, an which a thin film is formed; a heating section for heating the thin film; a radiation thermometer for measuring a surface temperature of the thin film; and a control section for controlling a temperature which is heated by the heating section, in response to the temperature measured by the radiation thermometer.
Also, to achieve the above object, the present invention according to a third aspect thereof further comprises, before the step of measuring the temperature of the thin film, steps of: measuring a surface temperature of a reference sample which is made of same material as the substrate and is set at a target temperature, by the radiation thermometer; and correcting a measurement value of the radiation thermometer, based on temperature data obtained by the step of measuring the temperature of the reference sample.
Also, to achieve to the above object, the present invention according to a fourth aspect thereof has a structure as follows. The heat treatment apparatus described above further comprises: a reference sample which is made of same material as the substrate and is set at a target temperature; and a correcting section for measuring a surface temperature of the reference sample, and for correcting a measurement value of the radiation thermometer, based on temperature data obtained by measurement of the surface temperature of the reference sample.
Preferred embodiments of the present invention will be as follows, for example.
(1) The substrate to be processed is a photomask blank in which a thin film containing metal made of chrome (Cr) or the like is formed on a transparent substrate made of quartz or the like, and a photosensitive thin film made of a chemical amplification type resist or the like is formed thereon.
(2) There is provided a blackbody for cutting stray light which enters into the radiation thermometer through the surface of the substrate.
(3) The blackbody is provided at a position optically symmetrical to the radiation thermometer with respect to a surface of the thin film, and the blackbody is set at a predetermined temperature.
(4) The radiation thermometer is an infrared sensor.
(5) The radiation thermometer makes measurement with use of light of a wavelength range except for light of a wavelength range which is radiated to the radiation thermometer from the substrate.
(6) The wavelength range of the light measured by the radiation thermometer is set to either a range of 2.7 to 2.8 xcexcm or a range from 4.3 xcexcm. Furthermore, the wavelength range of the light is desirably set to 9 xcexcm or more.
(7) The heating section is a halogen lamp or a hot plate. The heating section is provided in a side of a surface of the substrate which is opposite to another surface of the substrate where the thin film is formed, and applies energy to the substrate to be processed.
(8) A straightening plate, which controls the flow of a gas above the substrate and transmits the light of the wavelength range whose temperature is monitored, is provided above the substrate to be processed. The straightening plate is movable in the vertical direction, the lateral direction, and the height direction.
(9) The straightening plate exists on a light passage between the blackbody and the radiation thermometer and has a plurality of holes in the direction along the light passage.
(10) To approximate the temperature of the photosensitive thin film to a previously aimed temperature, an energy value required for heating or cooling is calculated from the measured temperature, and the energy value thus obtained is transmitted as a power for the heating or cooling means.
(11) When the processing amount calculated from the measured temperature and the heating processing time reaches a preset value, heating is stopped.
(12) After the energy for heating the substrate during measurement is shut off, the temperature is measured with use of the radiation thermometer. Correction of adding an offset or the like is made to the obtained temperature data, thereby to calculate the temperature.
(13) Modulation with a lower frequency than the response speed of the temperature measurement system is effected on the energy for heating the substrate to be processed.
(14) The reference sample is a sample in which a thin film having a radiation rate substantially equal to the photosensitive thin film with respect to the measurement wavelength of the radiation thermometer is formed on a substrate having the same material as the substrate to be processed.
(15) The reference sample has the completely same structure as the substrate to be processed.
(16) A resistance bulb or a thermocouple is embedded near the surface of the reference sample.
(17) The surface temperature of the reference sample is measured by the radiation thermometer or a resistance bulb, or a thermocouple, and there is provided a correcting section for correcting the measurement value of the radiation thermometer, based on the temperature data obtained by this measurement.
According to the structure described above, the present invention provides the following functions and advantages. The following result will be obtained in the case where a mask blank in which a thin film containing metal of chrome or the like is formed on a transparent substrate of quartz or the like and a photosensitive thin film having a thickness of about 500 nm is further formed thereon is used as a substrate to be processed on which a photosensitive thin film (photo resist film) is formed. The infrared transmittance of the mask blank is substantially zero within a range of 2.7 to 2.8 xcexcm and within a range of 4.3 xcexcm or more. This is because the thin film containing metal existing on the mask blank substantially shields most of the light of this wavelength range. Therefore, by setting the measurement wavelength of the radiation thermometer within a range of 2.7 to 2.8 xcexcm or a range of 4.3 xcexcor more, the radiation thermometer does not detect radiation light from a heating source existing in the side opposite to the thin film containing metal with respect to the substrate. Accordingly, the surface temperature (temperature of the photo resist film) of the substrate to be processed can be measured with high preciseness by the radiation thermometer.
Here, the photo resist film existing on the photomask blank has very high flatness on the resist surface and therefore has a mirror characteristic. Therefore, there is provided a mechanism (which is a blackbody having a constant temperature) for removing stray light in the direction including a position of a mirror image symmetrical to the radiation thermometer with respect to the temperature measurement position as a reference position. The radiation thermometer therefore receives two radiation lights, i.e., radiation light from the surface (the photosensitive thin film and the thin film containing metal) of the substrate to be processed and radiation light which comes from the black body and is reflected on the surface of the substrate. By previously measuring radiation rates of the black body and the photosensitive thin film on the substrate to be processed, it is possible to know the radiation amount which enters into the radiation thermometer from the black body. Therefore, as for the radiation from the surface of the substrate, mixture of stray light can be efficiently prevented if the radiation amount which comes from the black body and is reflected on the surface of the substrate is subtracted from the radiation amount which is actually received by the radiation thermometer. As a result of this, the temperature of the photosensitive thin film can be measured with high preciseness by using the radiation thermometer.
Also, the measurement sensitivity of the radiation thermometer changes depending an service conditions such as the environmental temperature or the like. Before processing the substrate the radiation thermometer used for measurement is used to measure the temperature of a reference sample, and correction is made to the measurement value of the radiation thermometer. Temperature measurement is thereafter carried out. Measurement errors among respective substrates to be processed can be thereby eliminated substantially. As a result of this, dimension controllability during PEB (Post Exposure Baking) is remarkably improved, and dimension errors among substrates to be processed can be reduced to be extremely small.
Also, if radiation based on lamp heating or the like is used as a heating means, the photo resist film can be heated without heating the quartz substrate as an under layer, within a particular wavelength range. In this manner, the processing time can be shortened. At this time, with respect to the mask blank on which a photosensitive material film is formed, the temperature measurement wavelength and the wavelength used for heating can be set to wavelengths different from each other, so that radiation required for heating does not become temperature measurement noise. Further, since a short processing time is enough, it is possible to reduce greatly occurrence of a heat distribution of the quartz substrate, and the uniformity in the plane of the substrate can be improved.
In addition, since heating is stopped when the processing amount calculated from the measured temperature and the heating processing time reaches a preset value, the total energy amount supplied to the substrate to be processed can be controlled strictly within the mask plane or between samples, so that controlability of absolute dimensions can be improved.
Also, in case where a radiation section is provided above the substrate to be processed with use of a lamp (radiation) as a heating means, it is considered that radiation noise may enter into the radiation thermometer. In this case, temperature measurement is carried out when the energy is OFF (or low), by shutting off the energy for heating the substrate or by modulating the energy with a lower frequency than the response speed of the temperature measurement system. In this manner, radiation noise can be greatly reduced by eliminating or reducing radiation caused due to heating only during temperature measurement. Accurate temperature measurement is enabled by making correction by software, e.g., by adding an offset value to the obtained temperature data.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.